In the field of hydro forming, it is known to inject the incompressible liquid in a heated preform made of plastic material at a pressure which is adapted to urge the wall of the preform against the wall of the molding cavity such that the preform is deformed and acquires the shape of the molding cavity and of the container to be produced.
However, it is also known that this pressure level is not sufficient to completely shape the preform into the container, meaning that, with the pressure applied to the preform, the preform acquires a shape which is not exactly the shape of the molding cavity and that an extra deformation is needed to completely urge the wall of the preform against the wall of the molding cavity. It is in particular the case when embossed letters or logos must be reproduced on the external surface of the container wall, or when the wall has ridges. Shapes having locally a very small radius of curvature are very difficult to obtain.
To this end, after the injection of liquid at a first pressure, a second pressure greater than the first pressure, is applied to the preform during a short time in order to create a pressure peak inside the preform, the pressure peak being arranged to finalize the shaping of the preform into a the container.
Several solutions have been proposed to apply the second pressure to the preform. FR-2 978 371 has disclosed an injection device comprising compression means between the liquid source and the outlet of the injection device. The compression means are arranged to temporarily increase the pressure of the liquid injected through the outlet of the injection. Consequently, when the compression means are not used, the liquid is injected at a first pressure and when the compression means are actuated, the pressure of the liquid is increased to provide the pressure peak needed to finish the forming of the container. A drawback of such a liquid forming method is that it needs two sealing mechanisms with their actuators, one between the compression means and the liquid pump and the other between the feeding duct and the preform neck.
US-2011/0135778 also discloses applying the pressure peak by temporarily varying the pressure of the liquid injected in the container. In this injection device, a stretch rod arranged to assist in the axial expansion of the container comprises an air vent placing the inner volume of the preform in fluidic communication with the atmosphere. Such an injection device does not allow precisely controlling the pressure inside the preform during its deformation since there is a pressure loss in the air vent of the stretch rod.
US-2013/0122136 discloses a device for injecting air and liquid into an neck of the container in order to form said container. Liquid is supplied by a pipe 30. A sealing mechanism 34 extends between the pipe 30 and the neck 20. Once the sealing mechanism closes the gap between the pipe and the neck, the container is still in fluidic connection with the liquid source through the pipe 30 and there is no movement of the pipe into the container.
According to another solution, WO-2012/037054 discloses a method wherein a liquid in injected at a first pressure in a preform placed in a molding cavity having a first volume. When the pressure peak is to be applied, the volume of the molding cavity is reduced by moving a part of the mold in order to increase the pressure applied to the preform. These solutions are satisfactory but require a modification of the injection device and/or of the mold and of the mold holder in order to carry out the step of increasing the pressure from the first pressure to the second pressure. The machine for producing containers is therefore made more complex and implementing these solutions on existing machines or providing a new machine is expensive and time consuming.
One of the aims of the invention is to overcome these drawbacks by proposing a method for producing containers wherein the second pressure can be applied and controlled in a simpler manner.